const new bindings for loops and function parameters

docs/const-binding-for-loops-and-function-parameters.md

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+# `const` bindings in for loops and function parameters
+
+## Summary
+
+Extend the existing `const` keyword to support per-binding const declarations in for loops variables and function parameters.
+
+## Motivation
+
+Luau already supports `const` for local bindings:
+
+```luau
+const x = 10
+const function foo() end
+```
+
+However, there is no way to mark loop variables or function parameters as immutable. This leads to accidental reassignment bugs, especially in generic for loops where key/value pairs should remain unchanged, and in functions where parameters should be treated as read-only.
+
+## Design
+
+### New syntax
+
+Generic loops:
+
+```luau
+for const k, const v in table do end   -- both const
+for const k, v in table do end         -- only k const
+for k, const v in table do end         -- only v const
+```
+
+Numeric loops:
+
+```luau
+for i = 1,10,1 do end -- normal usage
+for const i = 1,10,1 do -- const variable i
+    i *= 2 -- err
+end
+for const const = 1,10,1 do end -- const is constant variable --overwrite const keyword
+```
+
+Function parameters:
+```luau
+function foo(const x, const y) end     -- both const
+function bar(const x, y) end           -- only x const
+function baz(const, const x) end       -- const is not constant variable name --overwrite const keyword, x const
+```
+
+## Backward Compatibility
+
+```luau
+for const in x do -- const is not constant variable --overwrite const keyword -- If you used it this way before, there is no change.
+end
+
+for const const in x do -- const is constant variable --overwrite const keyword
+end
+
+for const = 1,10,1 do end -- if you used it this way before, there is no change
+
+function foo(const) do -- const is not constant variable --overwrite const keyword -- If you used it this way before, there is no change.
+end
+
+function foo(const const) do -- const is constant variable name --overwrite const keyword
+end
+```
+
+Rule: `const` is treated as a keyword **only when** the next token is a `Name` (and, in the `for` statement first-binding position, the token after that is not `=`))
+
+## Drawbacks
+
+- **Unusual syntax**: Normally, loops and parameter variables follow the pattern `name`, `name`, etc., but now you can add `const` before them.
+
+## Alternatives
+
+- **Do nothing**: Rely on linter rules or code conventions to catch accidental reassignment. This avoids parser complexity but provides no language-level guarantee.